Traditionally, many internal combustion engines are typically provided with an exhaust gas recirculation (“EGR”) system to recirculate a portion of the exhaust gas to an intake passage to control the emission and suppress the generation of NOx by lowering maximum combustion temperature and to improve fuel consumption by reducing pumping loss. In such EGR systems, a recirculation control valve for controlling the flow of the recirculation exhaust gas is typically positioned at or near the cylinder head of the internal combustion engine. Moreover, a recirculation exhaust gas inlet through which part of the exhaust gas flowing through an exhaust passage is extracted is formed in an exhaust manifold or an exhaust pipe of the engine.
Recently, in some engine designs, cooled EGR can be used to increase the fuel efficiency since the addition of cooled EGR substantially reduces the knock tendency of the engine, resulting in a potential to increase the engine compression ratio and an opportunity to improve combustion phasing and combustion cycle efficiency. Combining advanced combustion phasing with increased specific heat of the fuel-air mix results in a substantial decrease in combustion temperatures which reduces the need for a rich mixture at even the highest power levels.
A block diagram of an exemplary EGR system is shown in FIG. 1 (prior art). As shown, the EGR system includes a four-cycle engine 1 for automobiles, powered by the combustion of a gas mixture comprising fuel and air. Specifically, an intake pipe line 2 (i.e., an intake runner) is connected to the engine 1 and provided to supply air to the engine 1. Moreover, an intake manifold 10 is provided from which the intake pipe line 2 extends. As known to those skilled in the art, the intake manifold 10 receives air from the outside that is filtered by an air cleaner 3 (e.g., an air filter) to remove dust contained in the outside air as well as for feeding air to the intake line 2 via the intake manifold 10. Furthermore, a throttle valve 5 is provided for regulating the amount of air that is fed into the intake manifold 10 and a fuel injector 4 is provided on the intake line 2 for injecting fuel including gasoline into the engine 1. Alternatively, the engine 1 can have a fuel injector 4′ located at a position where fuel is injected directly to the combustion chamber or sub-combustion chamber. Further, an exhaust pipe line 6 is connected to the engine 1 to expel the exhaust gas generated by combustion in the engine 1, and a purifying apparatus 7 (e.g., a catalytic converter) is positioned at the opposing end of the exhaust line 6 to purify the exhaust gas before it is forced out of the tailpipe (not shown).
As further shown, engine 1 includes a combustion chamber 1a, an intake valve 1b for closing communication between the intake line 2 and the combustion chamber 1a, an exhaust-gas valve 1c for closing communication between the exhaust pipe line 6 and the combustion chamber 1a, and a piston 1d which moves vertically in the combustion chamber 1a during operation. The operation of engine 1 is known to those skilled in the art and will not be repeated herein. It should also be appreciated that while only one engine cylinder is shown, the engine configuration contemplated herein is for a four cylinder engine, V6 engine, V8 engine or the like and that the single cylinder is shown in FIG. 1 only for illustrative purposes.
The EGR components of the system include an EGR valve 8, an exhaust gas intake pipe line 15, an EGR pipe line 16 and a control unit 18. The exhaust gas intake pipe line 15 extends from the exhaust line 6 and is provided to transfer the exhaust gas to the EGR pipe line 16 to recirculate the exhaust gas to the intake manifold 10. Further the control unit 18 is provided to control the EGR valve 8 by outputting a control signal in response to the running state of the engine. Such controls can typically be based on engine operation conditions, including the temperature of engine coolant, the number of engine rotations and the degree of opening the injector (amount of fuel injection). Once the EGR valve is opened (or its positioned is changed) in response to the control signal, exhaust gas flows into the EGR pipe line 16 and returns to the engine combustion chamber 1a through the intake manifold 10 and the intake line 2. Consequently, combustion in the automobile four-cycle engine 1 is suppressed by the amount of non-flammable exhaust gas returned to the combustion chamber 1a. As discussed above, some designs include an EGR cooler 17 that can be provided on exhaust gas intake pipe line 15 to cool the exhaust gas before it is introduced into the intake manifold 10.
In conventional EGR systems, assuming an instantaneous responsive EGR valve, the EGR mass flow rate into the intake manifold 10 equals the total mass flow rate out of the intake manifold 10 into the engine 1 multiplied by the desired mass fraction in the intake manifold. The EGR mass flow rate can be mathematically described by equation (1) as follows:{dot over (m)}ie=Xde·{dot over (m)}ot  (1)where,                {dot over (m)}ie is the EGR mass flow rate into the intake manifold;        {dot over (m)}ot is the total mass flow rate out of the intake manifold and into the engine cylinder; and        Xde is the desired EGR mass fraction in the intake manifold.        
The desired EGR mass fraction Xde is a variable value that is determined by the engine manufacturer to maximize fuel consumption efficiency based on operating conditions. In order to achieve the desired EGR mass fraction Xde, the EGR control valve is electronically actuated by the control unit 18 to varying predefined positions to control the amount of exhaust gas that is recirculated back into the intake manifold 10 via the EGR pipe line 16. The position of the EGR control valve will vary depending on engine operating conditions as discussed above and as would be known to those skilled in the art.
Once the EGR control valve is actuated to a defined position and the exhaust gas is introduced into the intake manifold 10, the amount of exhaust gas (i.e., EGR mass) in the intake manifold 10 increases or decreases in proportion to the amount of air being introduced through the intake lines. The current EGR mass fraction in the intake manifold can be represented by equation (2) as follows:
                              X          ce                =                              m            e                                              m              u                        +                          m              a                                                          (        2        )            where,                Xce is the current EGR mass fraction in the intake manifold;        me is the current EGR mass in the intake manifold; and        ma is the current mass of air in the intake manifold.        
Based on the foregoing, it should be appreciated that the EGR mass flow out of the intake manifold {dot over (m)}oe is the current EGR mass fraction Xce multiplied by the total mass flow rate {dot over (m)}ot. This result can be represented by equation (3) as follows:{dot over (m)}oe=Xce·{dot over (m)}ot  (3)where,                {dot over (m)}oe is the current EGR mass flow rate out of the intake manifold;        {dot over (m)}ot is the total mass flow rate out of the intake manifold; and        Xce is the current EGR mass fraction in the intake manifold.        
When the engine is operating at a steady state, the current EGR mass fraction Xce will equal the desired EGR mass fraction Xde because the EGR flow rate into the intake manifold {dot over (m)}ie will be constant and equal to the EGR flow rate out of the intake manifold {dot over (m)}oe. However, during a transient cycle of the engine, for example when the engine load is increasing or decreasing during engine acceleration or deceleration, the EGR flow rate will be changing in response to a change in the throttle position. Generally, the rate of change of EGR mass in the intake manifold can be represented by equation (4) as follows:
                                          ⅆ                          m              egr                                            ⅆ            t                          =                                            m              .                        ie                    -                                    m              .                        oe                                              (        4        )            This equation is defined by the EGR mass flow rate out of the intake manifold {dot over (m)}oe subtracted from the EGR mass flow rate into the intake manifold {dot over (m)}ie. Thus, during engine acceleration, the throttle is open, which results in an increase in the mass flow rate out of the intake manifold {dot over (m)}ot and, therefore, a higher EGR mass flow rate into the intake manifold {dot over (m)}ie. Alternatively, during engine deceleration, the throttle is closed, which effectively decreases the total mass flow rate out of the intake manifold {dot over (m)}ot leading to a lower EGR mass flow rate into the intake manifold {dot over (m)}ie.
Next, the rate of change of EGR mass in the intake manifold can further be represented in terms of desired EGR mass fraction Xde and actual EGR mass fraction Xce by substituting the foregoing equations (1) and (3) into equation (4) to derive the following equation:
                                          ⅆ                          m              egr                                            ⅆ            t                          =                              (                                          X                de                            -                              X                ce                                      )                    ·                                    m              .                        ot                                              (        5        )            where,                Xde is the desired EGR mass fraction in the intake manifold;        Xce is the current EGR mass fraction in the intake manifold;        {dot over (m)}ot is the total mass flow rate out of the intake manifold; and        
      ⅆ          m      egr            ⅆ    t  is the current rate of change of EGR mass in the intake manifold.
As discussed above, when the engine is operating in a transient state, for example when the engine is accelerating or decelerating, engine manufacturers typically design engines to increase or decrease EGR flow to maximize fuel consumption efficiency. For example, as shown in FIG. 2 (prior art), at approximately the 17 second mark, the engine switches from a low load to a high load (i.e., engine acceleration), and the desired EGR flow rate EGR_des is designed to increase from slightly under 20 g/s to 40 g/s. As noted above, this is done by adjusting the position of the EGR control valve to increase the EGR flow rate into the intake manifold 10. Moreover, at approximately the 70 second mark as shown in FIG. 2, the engine load switches to a low load (i.e., engine deceleration) and the desired EGR flow rate EGR_des decreases to the original rate accordingly.
Although the change in desired EGR flow rate EGR_des is almost instantaneous, in actual operation in conventional EGR systems, there is a substantial delay in the actual EGR flow rate EGR_act from reaching the maximum desired rate of 40 g/s. This delay is also shown in FIG. 2 and is a result of the transport delay. In other words, although the EGR control valve opens instantaneously (or close to instantaneously) in response to the change in engine load, the amount of exhaust gas that is actually in the intake manifold 10 at that time is relatively low. Therefore, there is an inherent delay before exhaust gas is expelled into the intake pipeline 2 from the intake manifold 10 because it takes a given amount of time or number of engine cycles before the intake manifold 10 is filled with exhaust gas from EGR pipe line 16 and, thus, exhaust gas is also transferred from the intake manifold 10 into the engine 1. The delay is represented in FIG. 2 as shown for the actual EGR flow rate EGR_act. Moreover, it should be appreciated that the change in EGR mass in the intake manifold dm/dt is indirectly proportional to the derivative of the actual EGR flow rate EGR_act. Therefore, as the actual EGR flow rate EGR_act reaches its desired rate of 40 g/s, the change in EGR mass dm/dt reaches zero. Furthermore, at approximately the 70 second mark when the engine load switches back to a low load during engine deceleration, the change of rate of EGR mass in the intake manifold will be a negative value, before slowly returning to a zero value.
It is further understood to those skilled in the art that during operation of conventional internal combustion engines, and specifically after the exhaust stroke of the combustion cycle, that residual gases (containing combustion products and nitrogen) generally remain in the cylinder. Residual gas affects the engine combustion processes (and therefore emissions and performance) through its influence on charge mass, temperature and dilution.
In conventional EGR systems, the amount of excess residual gas that collects in the cylinder during a transient cycle of engine operation often increases above stable limits for combustion. For example, as shown in FIG. 3 (prior art), when the engine is operating at a high load with high intake pressure, the amount of residual gas that collects in the cylinder is relatively low because the high intake pressure forces the residual gas through the exhaust port during valve overlap of the exhaust stroke. Once the vehicle decelerates (i.e., decreased engine load), the intake pressure also decreases. As a result of this pressure drop, the concentration of residual gas in the cylinder increases dramatically and quickly. At the same time, the decrease in engine load, which is achieved by closing the throttle valve, also results in a reduction in the EGR flow to the intake manifold 10. However, because the intake manifold 10 is relatively full of exhaust gas at that time, there is a delay before the EGR out of the intake manifold 10 and into the combustion chamber of the engine is realized due to the transport delay discussed above with respect to FIG. 2. Effectively, the concentration of EGR into the cylinder decreases at a rate slower than the concurrent spike in the concentration of residual gas in the cylinder. The combination of EGR and residual gas in the cylinder (i.e., the burned gas) peaks above a stable limit for combustion resulting in operation condition problems, such as engine misfire. Accordingly, what is needed is an EGR control system and method that reduces transient delays for EGR flow rates caused by change in engine operating conditions.